1. Technical Field of the Invention
The present invention relates to the decoding of radiofrequency (RF) transmission channels conveying coded digital information. The invention thus applies advantageously to satellite digital telebroadcasting, for example, as defined in the DVB S (Digital Video Broadcasting-satellite) European specification based on the MPEG transmission standards, and using for example to convey the information, quadrature digital modulation. The invention also relates in particular to tuners, and also to demodulators and to the actual channel decoding processing.
2. Description of Related Art
Television signals originating from a satellite are amplified and converted into a predetermined frequency band (typically 950-2150 MHz) by way of a parabolic dish and a low noise converter which is located at the focus of the parabolic dish. This signal is then dispatched to the input of the tuner of the receiver. The purpose of the tuner is to select the desired channel and to output a baseband signal on the in phase path (I path) and on the quadrature path (Q path). This signal is then converted into a digital signal and demodulated. The channel decoding processing then also comprises a block which distinguishes the zeros from the ones, typically by means of majority logic, then performs the entire error correction, that is to say typically a Viterbi decoding, deinterlacing, Reed Solomon decoding and deshuffling. The channel decoding device outputs packets which are decoded in a conventional manner in a source decoding device in accordance with the MPEG standards so as to redeliver at the output the initial audio and video signals transmitted via the satellite.
At the input of the receiver, the signal received is composed of the collection of channels which are transmitted for the satellite and are transposed into the 950-2150 MHz frequency band. The overall power received is substantially equal to the mean power on a channel increased by ten times the Napierian logarithm of the number of channels. This signal possesses a considerable variation, of the order of 50 dBm.
At present, in receivers as a whole, the signal received at the input is normally filtered by a wideband type filter (whose passband is of the order of several hundred MHz) placed just after the low noise input amplifier, this being so as to avoid the saturation of the subsequent stages of the tuner (especially the controlled gain amplification stages as well as the mixers of the frequency transposition stage).
Furthermore, a first type of solution provides for the low noise amplifier and the wideband filters to be made outside the chip containing the controlled gain amplification stage as well as the frequency transposition stage. These filters, whose cutoff frequencies can be tailored by selecting the desired channel, are then embodied as discrete components such as “varicap” diodes. However, such components are of overly large size which is incompatible with a fully integrated embodiment of the tuner.
Moreover, in this type of solution, not only is the tuner not fully integrated, but it is made on a semiconductor substrate different from the substrate which supports the digital part (or digital domain) of the processing, namely the demodulation and the actual channel decoding. Stated otherwise, the demodulation and the channel decoding are carried out in a separate component from that integrating the tuner. Also, the tuner is generally screened so as to prevent the noise generated by the digital part from interfering in the mixing of the signals of the analog part. Thus, in this first type of solution, a front end device incorporated into a satellite television signals receiver, and capable of performing the tuning, demodulation and channel decoding, comprises several separate electronic components made on different chips.
Another type of solution has been envisaged and is described for example in French Patent Application No. 2,804,986. In this second type of solution, a unique “front end” electronic component is proposed, that is to say one embodied on a single chip, and integrating a radiofrequency tuner, a demodulator and a channel decoder.
It is turning out to be beneficial nowadays to be able to simultaneously output from the receiver several, that is to say at least two, MPEG data streams, corresponding respectively to different channels.
Now, if the first hardware solution mentioned above is adopted, such a receiver would then be composed of several identical elementary receivers, thereby leading to the production of a large number of different chips, so as to be able to incorporate the various different electronic packaging of each of the elementary receivers. This results in an increase in cost and a very considerable surface area.
If the second solution alluded to hereinabove is adopted, that is to say a unique electronic component, embodied on a single chip, it would then be necessary to produce on this unique chip, several completely separate tuners, capable of being controlled individually so as to select the various desired channels intended to be delivered simultaneously at the output of the receiver. However, embodying several completely separate tuners on one and the same chip would then lead to problems of coupling between the various voltage-controlled oscillators that are intended to generate the various mixing signals.
Thus, currently, the person skilled in the art, who wishes to produce a receiver of satellite digital television signals that is capable of simultaneously delivering several MPEG streams corresponding to several different channels, is confronted with a choice between two unsatisfactory solutions. Specifically, the first solution leads to the production of several different receivers produced on a considerable number of different chips, this being penalizing from a surface area and cost point of view. The other solution, more beneficial from a surface area point of view, poses technological problems of stray coupling.
There exists a need in the art for a solution to the foregoing problem.